1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an array substrate of an LCD device and a method of fabricating the same.
2. Discussion of the Background Art
In general, a liquid crystal display (LCD) device utilizes optical anisotropy and birefringence properties of liquid crystal molecules to display images. The LCD device usually has first and second substrates spaced apart from and opposing each other, and a liquid crystal layer interposed therebetween. The first and second substrates respectively have electrodes for forming an electric field between the electrodes. That is, if a voltage is applied to the electrodes of the LCD device, an electric field is formed between the electrodes and the electric field changes the alignment of the liquid crystal molecules. The changed alignment of the liquid crystal molecules controls a light transmittance through the liquid crystal layer and thus images can be displayed by controlling the light transmittance through the liquid crystal layer.
Among the various type of LCD devices commonly used, active matrix LCD (AM-LCD) devices have been developed because of their high resolution and superiority in displaying moving images. The AM-LCD device includes a thin film transistor (TFT) in each pixel region as a switching device, a pixel electrode in each pixel region, and a common electrode.
FIG. 1 is an exploded perspective view of an LCD device according to the related art. As shown in FIG. 1, an LCD device 20 has an upper substrate 22 having a black matrix 25, a color filter layer 26 and a common electrode 28 on the color filter layer 26. The LCD device further includes a lower substrate 12 having a thin film transistor (TFT) T and a pixel electrode 18 connected to the TFT T. The color filter layer 26 includes red, green and blue color filters 26a, 26b and 26c. 
A liquid crystal layer 30 is interposed between the upper and lower substrates 22 and 12. The lower substrate 12 is referred to as an array substrate because array lines including a gate line 14 and a data line 16 are formed thereon. The gate line 14 and the data line 16 cross each other, and the TFT T as a switching element is disposed in a matrix and is connected to the gate line 14 and the data line 16. The gate line 14 and the data line 16 cross each other to define a pixel region P. The TFT T is formed near the crossing portion of the gate line 14 and the data line 16. The pixel electrode 18 is formed of a transparent conductive material in the pixel region P. The upper substrate 22 is referred to as a color filter substrate because the color filter layer 26 is formed thereon.
The upper and lower substrates 22 and 12 are attached with a seal pattern (not shown) through a liquid crystal cell process. The seal pattern keeps a cell gap of the LCD device 20 uniform and prevents liquid crystal materials in the space between the upper and lower substrates 22 and 12 from leakage. Although not shown, the upper and lower alignment layers are formed at boundaries between the liquid crystal layer 30 and the upper and lower substrates 22 and 12, respectively, wherein the upper and lower alignment layers can improve reliability for alignment of the liquid crystal layer 30. In addition, the LCD device 20 includes at least one polarizer (not shown) on or under an outside surface thereof, and a backlight unit (not shown) may be disposed under the LCD device 20 as a light source.
An image signal transmitted by the data line 16 is applied to a predetermined pixel electrode 18 by sequentially scanning ON/OFF signals of the TFT T to the gate line 14. Hence, the liquid crystal layer 30 is driven by a vertical electric field, and images are displayed based on the change of the light transmittance thereof.
The base substrate of the LCD device generally has been made of a transparent glass substrate. Recently, a plastic substrate, which is lighter and more flexible than the glass substrate, is suggested as a base substrate of the LCD device for a small portable display device such as a notebook and personal digital assistants (PDA).
However, since the plastic substrate is more susceptible to heat and chemical treatment than the glass substrate, it is difficult for the LCD device to adopt the plastic substrate as the base substrate because a process of manufacturing an array substrate is usually performed under a high temperature more than about 200 degrees Celsius. Further, it usually requires several high temperature processes when manufacturing the array substrate. Therefore, to solve the problem, a color filter substrate without the array elements may be made of the plastic substrate, but the array substrate is still usually made of the glass substrate.
Another solution is using small molecule organic material and applying a low temperature process less than about 200 degrees Celsius so that the flexible plastic substrate can be used for manufacturing the array substrate.
Hereinafter, a method of fabricating the array substrate of the LCD device using the flexible plastic substrate under a low temperature of less than about 200 degrees Celsius will be described. It should be noted that although a metal layer, an insulating layer and a passivation layer do not affect the characteristics of the thin film transistor in the low temperature process, when a semiconductor layer including a channel region of the thin film transistor is made of a general semiconductor material under the low temperature process, an electric property of the thin film transistor will be affected because the semiconductor layer has a weak inner structure under the low temperature process and conductivity of the semiconductor layer is reduced in comparison with the semiconductor layer under the general temperature process.
To solve the problem, the semiconductor layer is made of an organic semiconductor material, wherein the organic semiconductor material includes a small molecule material and a polymer material. Here, the small molecule organic semiconductor material has higher conductivity than the polymer organic semiconductor material.
FIG. 2 is a schematic cross-sectional view of an array substrate including a semiconductor layer of a small molecule organic semiconductor material for an LCD device using a flexible plastic substrate according to the related art. In FIG. 2, a gate line (not shown) and a gate electrode 53 are formed by depositing and patterning a metallic material on a plastic substrate 50. A gate insulating layer 57 is then formed by coating an organic insulating material over an entire surface of the substrate 50 including the gate line and the gate electrode 53.
A semiconductor layer 60 is formed by evaporating the small molecule organic semiconductor material such as Pentacen (C22H14) over the gate electrode 53. Since the small molecule organic semiconductor material such as Pentacene is in a powder form and is difficult to be made in a solution form, it is difficult to deposit Pentacene by a chemical vapor deposition (CVD) process and pattern Pentacene by the photolithography process in which Pentacene would contact a photoresist material having moisture, a development solution and a stripping solution. Accordingly, the semiconductor layer 60 is evaporated by using a shadow mask 70 having an opening portion (not shown). However, this process imposes a limitation with respect to a width W1 of the semiconductor layer 60 and a distance W2 between the semiconductor layers 60.
More specifically, the shadow mask 70 is made of a metallic material, wherein a width of the opening portion corresponding to the width W1 of the semiconductor layer 60 should be at least more than about 40 micrometers. That is, the width of the opening portion should be at least more than about 40 micrometers, and the distance between the opening portions corresponding to the distance W2 between the semiconductor layers 60 should be more than about 120 micrometers. This is because diffusion of the material should be taken into account in the evaporation process.
As a result, the width W1 of the semiconductor layer 60 is at least more than 40 micrometers, wherein a length of a channel for the regular semiconductor layer 60 is less than about 10 micrometers. Reducing a size of the semiconductor layer 60 except the channel region helps to increase the aperture ratio. The more the pixel region is increased, the more the resolution is increased, and the less the size of the pixel region is reduced. Accordingly, the less the size of the thin film transistor in the pixel region is reduced, the less the size of the channel is also reduced. Accordingly, the small molecule organic semiconductor layer 60 using the shadow mask 70 is not suitable for a high-resolution array substrate because the width W1 of the semiconductor layer 60 is at least more than about 40 micrometers.
Further, when the source and drain electrodes are formed later by the photolithography process using a development solution and a stripping solution, those solutions will damage the semiconductor layer 60 under the source and drain electrodes. Therefore, it is difficult to apply the small molecule organic semiconductor layer in an array substrate of the high resolution LCD.